This invention relates to the reproduction of a digitally encoded signal from a medium and in particular to the identification of replay data in a multiplex format.
The introduction of disks recorded with digitally compressed audio and video signals, for example, utilizing MPEG compression protocols, offers the consumer sound and picture quality virtually indistinguishable from the original material. However, consumer users will expect such digital video disks or DVDs to offer features similar to those of their analog video cassette recorder or VCR. For example, a VCR may reproduce in either forward or reverse directions at speeds other than the recorded speed. Such non-standard speed playback features are also known as trick play modes. The provision of trick play features are less easily provided with MPEG encoded video signals due to the hierarchical nature of the compression which forms pictures into groups having varying degrees of compression. These groups are termed groups of pictures or GOPs, and require decoding in sequence. A detailed description of the MPEG 2 standard is published as ISO/IEC Standard 13818-2. However, in simple terms, an MPEG 2 signal stream may comprise three types of pictures having varying degrees of content compression. An intra-coded frame or I frame has the least compression of the three types and may be decoded without reference to any other frame. A predicted frame or P frame is compressed with reference to a preceding I or P frame and achieves greater degree of compression than an intra-coded frame. The third type of MPEG frame, termed a bi-directionally coded or B frame, may be compressed based on predictions from preceding and/or succeeding frames. Bi-directionally coded frames have the greatest degree of compression. The three types of MPEG frames are arranged in groups of pictures or GOPs. The GOP may for example contain 12 frames arranged as illustrated in FIG. 1A. Since only an intra-coded frame is decodable without reference to any other frame, each GOP may only be decoded following the decoding of the I frame. The first predicted frame or P frame, may be decoded and stored based on modification of the stored, preceding I frame. Subsequent P frames may be predicted from the stored preceding P frame. The prediction of P frames is indicated in FIG. 1A by the curved, solid arrow head lines. Finally, bi-directionally coded or B frames may be decoded by means of predictions from preceding and or succeeding frames, for example, stored I and P frames. Decoding of B frames by predictions from adjacent stored frames is depicted in FIG. 1A by the curved, dotted arrow head lines.
The hierarchical nature of the coded frames comprising MPEG groups of pictures necessitates that the I and P frames of each GOP are decoded in the forward direction. Thus, reverse mode features may be provided by effectively jumping back to an earlier, or preceding I frame and then decoding in a forward direction through that GOP. The decoded frames being stored in frame buffer memories for subsequent read out in reverse to achieve the desired reverse program sequence. FIG. 1B illustrates play back in the forward direction at normal speed and at a time prior to time to, a reverse three times speed mode trick play mode is selected. The trick play mode is initiated at time to where I-frame I(25) is decoded and displayed. The next frame required for decoding is I frame I(13), thus the transducer is repositioned, as indicated by arrow J1 to acquire frame I(13). Having recovered and decoded I-frame I(13), the transducer tracks, as indicated by arrow J2 to acquire and decode frame P(16). The process is repeated as indicated by arrows J3, J4. Following the acquisition and decoding of frame P (22) the transducer is moved as depicted by arrow Jn to recover frame I (1). To smoothly portray scene motion requires the decoding and display of I, P, and possibly B-frames. The jump and play process is repeated for preceding GOP, thereby progressing haltingly backwards through the records whilst smoothly portraying the program material in a reverse sequence at the video output.
The provision of visually smooth reproduction during trick mode reproduction requires timely disk retrieval and access to specific pictures from memory. Although each digital disk is encoded with navigation data which provides picture access points within each video object unit, these are limited in number, and may inherently contribute to temporally aliased image motion. To achieve temporally smooth trick mode reproduction, at multiple speeds in forward and reverse directions requires access to, and decoding of all encoded pictures. Although such performance is attainable at the cost of storage capacity, bit stream analysis and selection for buffer storage provides opportunities for improved trick mode reproduction through efficient memory utilization.
In an inventive arrangement, an apparatus reproduces a digitally encoded signal from a medium. The apparatus comprises a transducer for transducing the digitally encoded signal and generating a bit stream therefrom. A processor is coupled to receive the bit stream for controllably processing the bit stream. A memory is coupled to the processor for storing the processed bit stream information. A controller is coupled to control the memory and the processor for controlling identification of information within the bit stream, wherein the controller controls the processor to identify a specific sector type in the bit stream and responsive to the specific sector identification the controller controls the memory for storing the identified specific sector.
In an inventive method a start code is acquired from a plurality of start codes in a data stream arranged in a plurality of sectors transduced during reproduction by a digital disk apparatus. The method comprises the steps of searching the data stream to locate a specific sector type in the plurality of sectors. The specific sector type is searched to locate a start code from the plurality of start codes. The start code is tested to determine if it is incomplete. Searching the data stream to locate a second sector of the specific sector type in the plurality of sectors. Searching the second sector of the specific sector type to locate a second start code of the start code type from the plurality of start codes. Determining if the second start code is the residue of the incomplete start code. Combining the incomplete start code value and the residue start code value to form a complete start code.
A further inventive apparatus for reproducing a digitally encoded signal from a medium, comprises a transducer for transducing the digitally encoded signal and generating a bit stream therefrom. A first memory is coupled to the transducer for storing the bit stream. A second memory for storing data is controllably coupled from the first memory. A controller is coupled to control the first and second memories for controlling identification of information within the bit stream wherein the controller controls the first memory to output the bit stream from a specific sector address therein, and the controller controls the second memory to store a first part of the bit stream output from the specific sector address.
In another inventive apparatus a digitally encoded signal is reproduced from a medium. The apparatus comprises a transducer for transducing the digitally encoded signal and generating a bit stream therefrom. A memory is coupled to the transducer for storing the bit stream. A processor is coupled to the memory for processing the stored bit stream to identify an MPEG start code contained therein. The processor searches the stored bit stream to identify the MPEG start code in the stored bit stream and responsive to the MPEG start code identification the processor indicates the identification and stores a sector address of the identified MPEG start code.